Many computing devices include displays for displaying various types of images. Examples of such displays include cathode ray tubes (CRTs) liquid crystal displays (LCDs), electrophoretic displays (EPDs), light emitting diode displays (LED displays), and the like. Different types of displays have different components, configurations, and principles of operation for converting digital image data into the displayable image and displaying the image. An LCD, for example, uses the properties of liquid crystal molecules to control light passing through crossed polarizers. Several different mechanisms can add color to the light. Typically in computing devices, the LCD includes a layer of pixels arranged in a two-dimensional matrix. Each pixel is composed of three subpixels, one each allowing only red, green, or blue light to pass through. A layer of thin-film transistors (TFTs) controls the pixels, allowing a percentage of the incident light through each subpixel to generate the perceived colors of the pixels.
LCDs and some other types of displays do not generate light and must be illuminated by an optical system. Some optical systems include a light-emitting diode (LED) array, which converts an applied voltage into emitted light; an assembly of light guides and diffuser panels receives the light and creates a uniform luminance across the display. Such optical systems can be made substantially flat and used in flat-panel monitors and mobile devices. In particular, mobile devices may use an LED-backlit LCD in which the optical system emits light onto the back surface of the LCD, and the light passes through the LCD to produce the picture on the viewable surface. Light emission and power consumption is controlled by pulse-width modulation (PWM) of the voltage, i.e., switching the voltage on and off at a consistent pulse frequency. Reducing the pulse frequency (i.e., increasing the pulse width) reduces the power consumed by the LED array. Further, “dimming” the screen on such devices technically consists of reducing the pulse frequency, causing the LED array to emit less light and the luminance of the LCD, also referred to as the “brightness” of the picture, to decrease.
The pulse frequency for an LED array at full luminance is around 200 Hz, whereby the intermittent light appears to a viewer to be steadily on. Other PWM optical systems also use pulse frequencies over about 75 Hz, which is considered the human “flicker fusion threshold” at which intermittent stimulus appears steady. However, as the screen is dimmed, the pulse frequency can approach a rate at which the viewer might detect flicker. Computing devices avoid this with a system limit on the minimum pulse frequency or maximum pulse width; consequently, the screen can only be dimmed to the minimum system brightness imposed by the limit.
Biological research has shown that exposure to visible light in the blue wavelengths—from about 450 nm to about 490 nm (for reference, the RGB color model “blue” is at about 450 nm)—can negatively affect brain patterns such as the circadian rhythm, which in turn may disrupt or decrease the benefits of sleep. Unfortunately, computing device displays emit light that includes a significant blue component. LEDs, in particular, are manufactured in a manner that causes the color spectrum of the emitted light to inherently feature a blue wavelength luminance that is significantly higher than the luminance of any other color. Software applications are available that attempt to address the issue by changing the color temperature of the picture, wherein some selectable color temperatures suppress blue light. However, simply changing the color temperature can decrease contrast or readability of the picture. These negative effects can be exacerbated when the screen is also dimmed, as is typical for night-time use of the computing device. Depending on the device and implementation, other image processing may be needed to maintain picture quality.